1. Field of the Invention
The present invention relates to signal processing, and more specifically, to modulation techniques, such as orthogonal frequency division multiplexing, used in signal transmission and reception.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a signal processing technology well known in the field of communications. In general, OFDM operates by dividing a frequency spectrum into smaller frequency subcarriers (a.k.a., tones) and modulating the subcarriers with parts of a data signal.
FIG. 1 shows a simplified block diagram of one implementation of a prior-art OFDM transmitter 100. Transmitter 100 has data symbol mapper 102, which receives a serial bitstream of digital data from upstream processing. The serial bitstream is divided into groups of bits, and each group is mapped into one or more data symbols to generate a serial stream of data symbols. Mapping may be performed using one or more suitable techniques such as quadrature phase-shift-keying (QPSK) and quadrature amplitude modulation (QAM).
Serial-to-parallel (S/P) converter 103 converts the serial stream of data symbols received from data symbol mapper 102 into D parallel streams of data symbols. Subcarrier mapper 104 assigns the D parallel data symbol streams to N subcarrier frequencies (i.e., tones), where the N subcarrier frequencies are arranged orthogonally to one another. In particular, each parallel data symbol stream is assigned to a separate output of subcarrier mapper 104, where each output corresponds to a different one of the N subcarriers. Note that, according to various implementations, a number D of data symbols and a number P of pilot symbols may be assigned to the N subcarriers, where there may be a number U of unused (i.e., free) subcarriers, such that N=D+P+U. The N outputs of subcarrier mapper 104 (i.e., Z=Z1, . . . , ZN) are then provided to inverse fast Fourier transform (IFFT) processor 106. IFFT processor 106 transforms each set of N outputs from subcarrier mapper 104 into one OFDM symbol, comprising N time-domain samples (e.g., z=z1, . . . , zN).
After IFFT processing 106, cyclic-prefix inserter (CPI) 108 inserts a cyclic prefix onto each OFDM symbol. A cyclic prefix, which is typically a copy of the last V samples of an OFDM symbol, enables the receiver to establish the timing of the OFDM symbol. Each OFDM symbol and each corresponding cyclic prefix are converted from parallel to serial format by parallel-to-serial (P/S) converter 110. The output of P/S converter 110 may be further processed using digital-to-analog (D/A) conversion, radio-frequency (RF) modulation, amplification, or other processing suitable for preparing an OFDM signal, comprising one or more OFDM symbols, for transmission.
FIG. 2 shows a simplified representation of one implementation of a prior-art OFDM signal 200 transmitted over a single-path channel (e.g., such as an additive white Gaussian noise (AWGN) channel). OFDM signal 200 comprises multiple OFDM symbols, including OFDM symbol 1 and OFDM symbol 2, where each OFDM symbol comprises N samples. A cyclic prefix is appended to the beginning of each OFDM symbol, where the cyclic prefix represents a copy of the last V samples of the corresponding OFDM symbol. For example, cyclic prefix 1, appended to OFDM symbol 1, is a copy of the last V samples of OFDM symbol 1 and cyclic prefix 2, appended to OFDM symbol 2, is a copy of the last V samples of OFDM symbol 2.